1. Field of the Invention
The present invention relates to degaussing coils and in particular, to a method and apparatus for securing a degaussing coil onto a cathode ray tube (CRT).
2. Description of the Prior Art
CRTs in general rely upon either electromagnetic or electrostatic deflection to scan an electron beam across a phosphor screen. Upon impact by electrons, the phosphors within the screen emit visible light in one or more colors. Large screen CRTs used in direct view television sets rely upon electromagnetic deflection generated by a yoke that is affixed to the neck of the CRT near the junction of the neck and the funnel of the CRT envelope.
Because certain components within the CRT, such as an inner magnetic shield, an aperture grill and a frame for the aperture grille, may become magnetized because of (1) magnetic fields generated by the yoke, or (2) other components within a television set such as permanent magnet loudspeakers, or (3) external accessories mounted on top of or near the television, such as video cassette recorders and video games, it is extremely desireable to periodically degauss the CRT in order to minimize distortion of an image created by scanning an electron beam across the phosphor screen of the CRT. In addition, heat shrink bands, as detailed further herein, are fabricated from a magnetic steel and therefore may also become magnetized.
An early degaussing method consisted of the use of an external degaussing coil which, when energized by an alternating current (typically at a frequency of 60 Hz) was manually moved around the envelope of a CRT. Since magnetic distortion is more noticeable with color CRTs, automatic degaussers were developed as color televisions became more popular. One type of automatic degausser consists of a coil wrapped about the perimeter of the faceplate of a CRT, where the coil is driven during a short period by a 60 Hz current in series with a device known as a "posistor." The posistor not only limits the amount of current flowing through the degaussing coil, but also limits the period during which current will flow. The posistor is initially conductive when voltage is applied to the circuit, and then becomes non-conductive when the temperature of the posistor reaches a predetermined temperature. The posistor remains non-conductive until voltage is removed from the circuit. Thus, the degaussing coil is always energized for a brief period commencing with initial power-on of a television set. To avoid a distorted initial image, the degaussing coil is de-energized before an image appears on the phosphor screen of the CRT.
Some television sets utilize a large external magnetic shield surrounding the funnel of a CRT. In such sets the degaussing coil is attached to the external magnetic shield by plastic clips that snap into holes in the shield.
In order to both reduce expense and improve performance, many CRTs include an internal magnetic shield. With such arrangements, since an external magnetic shield is not available for attachment of a degaussing coil, one prior art method of securing a degaussing coil to a CRT relies upon adhesive backed plastic holders applied to the glass surface of a CRT to thereby provide an anchor for the degaussing coil. Such plastic holders however, provide inconsistent attachment strength due to variations in CRT surface preparation. In particular, the funnel of a CRT is usually coated with carbon for grounding purposes. In addition, because of the curvature of the envelope of a CRT, mismatches between the adhesive surface of such holders and the CRT decrease adhesive strength.
Another prior art method of securing a degaussing coil to a CRT uses adhesive tape applied over the top of the degaussing coil, with the adhesive tape attached to the CRT on both sides of the degaussing coil. However, with such a method the level of adhesion is dependent upon the method of application of the adhesive tape. In particular, if the tape contacts the degaussing coil only at a tangent to the degaussing coil, the adhesive strength will be less than when the tape is in contact with a greater surface area (per length) of the degaussing coil.
With both adhesive methods, not only is the strength of the bond initially inconsistent, but the strength of the bond tends to decrease with repeated thermal cycling.
Yet another prior art method of securing a degaussing coil to a CRT uses plastic hooks that are attached to a heat shrink band which is wrapped around the perimeter of the faceplate of a CRT (metallic heat shrink bands provide implosion protection and are designed to meet the standards set by Underwriters Laboratories). The length of these plastic hooks are selected to position the degaussing coil in the center area of the CRT. Such hooks however are cumbersome to use and are expensive.
During operation of a television, since an anode cap of a CRT has a potential in the tens of thousands of volts, if the degaussing coil is somehow positioned too close to the anode cap, arcing may result. If on the other hand the degaussing coil is positioned too close to the deflection yoke, it becomes very difficult to attach magnets which are necessary to adjust the CRT during assembly and alignment of a television.
Therefore, it would be desireable to provide a method and apparatus for securing a degaussing coil to a CRT which permits the secure and precise positioning of the degaussing coil without the need for adhesives.